1. Field of the Invention
The present invention relates to semiconductor variable capacitance element for controlling a capacitance by an electric charge stored in a floating electrode covered by an insulating film and insulated from the outside, the floating electrode being disposed on the surface of a semiconductor substrate through the insulating film.
2. Description of the Related Art
A semiconductor variable capacitance element has been known as disclosed in a literature "Proceedings vol. 2, 11th INTERNATIONAL CONGRESS of CHRONOMERY, edited by the French Society of Michrotechnology and chronometry, p. 9, '84".
FIG. 3 is a sectional view showing the conventional semiconductor variable capacitance element. As shown in FIG. 3, the semiconductor variable capacitance element comprises a p-type semiconductor substrate 31, a floating electrode 33 disposed on and electrically insulated from the semiconductor substrate for storing electric charge to form a depletion layer capacitive region 30 in the semiconductor substrate, a charge control electrode 37 formed in a portion of the semiconductor substrate for injecting electric charge into the floating electrode, and a capacitance electrode 35 formed in a portion of the semiconductor substrate and capacitively coupled to the floating electrode 33. The floating electrode is covered by an insulating film 32 and insulated from the outside. The charge control electrode 37 is composed of a heavily doped P.sup.+ region in an n-well diffusion region. The capacitance electrode 35 is composed a heavily doped n.sup.+ region in an n-well diffusion region 34. Resistances 38 and 39 are protecting resistances for protecting the insulating film 32 from electrostatic discharge. The semiconductor variable capacitance element has a capacitance value between the heavily doped n.sup.+ region 35 and the semiconductor substrate 31. The capacitance value of the semiconductor variable capacitance element changes dependent on the electric potential of the floating electrode produced by the electric charge accumulated in the floating electrode 33. A maximum capacitance value thereof is determined based on an area of the floating electrode and an area of the capacitance electrode and a minimum capacitance value thereof is determined based on the areas and a minimum value of the depletion layer capacitance 30, i.e., a maximum value of a width of the depletion layer.
In the semiconductor variable capacitance element, the depletion layer capacitance 30 produced on the surface of the semiconductor substrate under the floating electrode 33 is controlled by the electric potential of the floating electrode 33 produced by the electric charge accumulated in the floating electrode 33. The floating electrode 33 and the capacitance electrode 35 have a strong capacitive coupling so that the electric potential of the floating electrode 33 is affected by a voltage of the capacitance electrode 35.
Conventionally, since a terminal 36 of the capacitance electrode 35 is connected to an external circuit directly, a bias voltage from the circuit is directly applied to the capacitance electrode 35. Thus, there is a drawback that when the capacitance value is changed by a change in the bias voltage from the circuit or when the bias voltage from the circuit is high, a high voltage is applied to the p-type diffusion region 35 and the floating electrode 33 so that the accumulated electric charge is changed by a fine tunnel current flowing to the floating electrode 33 and the capacitance value is also gradually changed with aging.
Semiconductor variable capacitance elements as shown in FIG. 3 are disclosed in the commonly assigned following applications, which are incorporated herein by reference: Ser. No. 008,290 filed Jan. 29, 1987, now U.S. Pat. No. 4,816,894; and Ser. No. 036,285 filed Apr. 9, 1987. The semiconductor variable capacitance elements perform analog tuning of the capacitance value by utilizing a floating gate memory device.